Low pressure neural contact structure

ABSTRACT

A low-pressure neural contact structure for contact with neural tissue, for example, neural tissue of the retina within which are ganglion cells to be electrically stimulated. The contact structure comprises a first portion for attachment to a first bodily location, such as the inner surface of the retina, and a second portion interconnected with the first portion via an interconnection and being held in contact with the neural tissue. The interconnection exhibits a weak restoring force which in conjunction with the geometry of said second portion provides a preselected desired pressure of contact against the neural tissue. As adapted for the retina, the interconnection exhibits a weak restoring force developed in response to curvature of the interconnection along the inner radius of the retina.

This is a continuation of application Ser. No. 07/943,513 filed on Sep.11, 1992 now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to prosthetic devices for interfacing with neuraltissue.

Neural tissue can be artificially stimulated and activated by prostheticdevices which pass pulses of electrical current through electrodes onsuch a device. The passage of current causes changes in electricalpotentials across neuronal membranes which can initiate neuron actionpotentials, which are the means of information transfer in the nervoussystem. Based on this mechanism, it is possible to input informationinto the nervous system by coding the information as a sequence ofelectrical pulses which are relayed to the nervous system via theprosthetic device. In this way, it is possible to provide a variety ofartificial sensations including touch, hearing, and vision for a varietyof applications. It is also possible to monitor and record neuralactivity using such a scheme.

One typical application of neural tissue stimulation is in therehabilitation of the blind. Some forms of blindness involve selectiveloss of the light sensitive transducers of the retina. Other retinalneurons remain viable, however, and may be activated in the mannerdescribed above by placement of a prosthetic electrode device on theinner (toward the vitreous) retinal surface. This placement must bemechanically stable, minimize the distance between the device electrodesand the neurons, and avoid undue compression of the neurons.

The retina is extraordinarily fragile. In particular, retinal neuronsare extremely sensitive to pressure; they will die if even a modestintraocular pressure is maintained for a prolonged period of time.Glaucoma, which is one of the leading causes of blindness in the world,can result from a chronic increase of intraocular pressure of only 10 mmHg. Furthermore, the retina, if it is perforated or pulled, will tend toseparate from the underlying epithelium, which will eventually render itfunctionless. Thus attachment of a conventional prosthetic retinalelectrode device is not practical, primarily because of the typicallyhigh pressures that such a device would exert on the retina, which wouldinevitably compromise the retinal neurons.

SUMMARY OF THE INVENTION

In general, in one aspect, the invention provides a low-pressure neuralcontact structure for contact with neural tissue. This structurefeatures a first portion for anchoring to a bodily location, and asecond portion interconnected with the first portion via aninterconnection and contacting the neural tissue. The interconnectionbetween the first and second portions exhibits a weak restoring forcewhich in conjunction with the geometry of the second portion provides apreselected desired pressure of contact against the neural tissue. Theinvention is adaptable for any neural stimulation or sensing applicationin which neural tissue surface contact is required, and particularlywhere low-pressure neural contact is required. Furthermore, thestructure provides the ability for attachment to neural tissue in alocation distant from an active location, thereby isolating anyphysiological degradation due to the attachment. And of equalimportance, the contact structure inherently provides adequate surfacepressure for contacting neural tissue, while at the same time minimizingthat pressure due to its ability to easily conform to contours in neuraltissue.

In preferred embodiments of the invention, the second portion providesmechanical support for a stimulating electrode structure positioned onthat portion for stimulating neurons within the neural tissue. The firstand second portions may together comprise an integral structure, or inother embodiments, may comprise distinct structures. The integralstructure is preferably of a homogeneous material and rectangular. Morepreferably, the integral structure is a silicon cantilever.

In other preferred embodiments, the neurons to be stimulated areautonomic nervous tissue associated with bladder function or neuronsassociated with activation of paretic limbs.

In still other preferred embodiments, the second portion comprisesmechanical support for a transducing electrode structure positioned onthe second portion for sensing neuronal activity, or for sensingchemical or physical parameters of the neural tissue. Preferably, thetransducing electrode senses oxygen or temperature. In another preferredembodiment, the second portion provides mechanical support for astructure adapted to disperse chemicals into the neural tissue.

In general, in another aspect, the invention provides a low-pressureneural contact structure for contact with neural tissue of the retinawithin which are ganglion cells to be electrically stimulated. Theinvention features a first portion for attachment to a first location onthe inner surface of the retina, and a second portion interconnectedwith the first portion via an interconnection and being held in contactwith a second location on the inner surface of the retina adjacent tothe ganglion cells to be stimulated. The interconnection exhibits a weakrestoring force developed in response to curvature of theinterconnection along the inner radius of the retina, whereby the weakrestoring force, in conjunction with the geometry of the second portion,provides a preselected desired pressure of contact against the retinalneural tissue.

In preferred embodiments, the contact structure comprises a siliconcantilever; the second portion provides mechanical support for astimulating electrode or array of electrode for stimulating the ganglioncells. Preferably, a layer of biocompatible encapsulation materialencapsulates the cantilever; more preferably, the encapsulation materialis silicone. The encapsulation material overhangs edges of thecantilever and is thicker in the location of the first portion than inthe location of the second portion. The encapsulation layer preferablyis perforated at the locations of the cantilever edges and provideschannels located at positions corresponding to the positions ofelectrodes on the cantilever.

In other preferred embodiments, the cantilever is between 2-40 μm-thick,2 mm-5 cm-long, and 0.5 mm-1 cm-wide, and the encapsulation layer isbetween 5-25 mm-thick. In other embodiments, a silicone rib ispositioned on the cantilever to bend the cantilever in a predeterminedgeometric shape. In still other embodiments, the width of the cantileverin the location of the first portion and in the location of the secondportion is greater than the width of the cantilever in the location ofthe interconnection.

Thus, the contact structure provides both a neural tissue contactingsurface and mechanical support for stimuli and sensory electronics andelectrodes. The structure is applicable to a wide range of stimulationprosthetics for visual, auditory, and sensory systems. Most importantly,the invention achieves the ability to contact neural tissue using anattachment scheme that does not interfere with an area of tissue to bestimulated, and yet at the same time provides adequate contact pressureagainst that tissue.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a is a view of the neural contact of the invention in place on theretina.

FIG. 1b is another view of the neural contact of the invention in placeon the retina.

FIG. 2a is a planar view of one embodiment of the neural contact of theinvention.

FIG. 2b is a side view of one embodiment of the neural contact of theinvention.

FIG. 3 is a planar view of the neural contact of the invention includingan encapsulation layer.

FIG. 4 is a side view of the neural contact of the invention includingan encapsulation layer.

FIG. 5 is a side view of a second embodiment of the neural contact ofthe invention.

DESCRIPTION OF A PREFERRED EMBODIMENT

Referring to FIGS. 1a and 1b, there is shown the low-pressure neuralcontact of the invention 10 adapted for stimulation of retinal neuralcells. As shown in the figure, the retina 12 consists of ten layers ofcells. The outermost layers 14 (away from the geometric center of theeye) contain the rods and cones, which are the cells that sense thepresence of light and initiate a nerve signal that passes to the brain.The innermost layers 16 (adjacent to the vitreous humor) primarilycontain the ganglion cells, which have axons extending into the brainvia the optic nerve 18. Between the inner and outer retinal layers aremany different cell types that process neural signals from the rods andcones before the signals are sent to the brain. Light coming from thefront of the eye must traverse the inner retina in order to reach therods and cones. The corresponding signals generated by the rods andcones then travel to the inner retina on the way to the brain.

There are two types of retinal diseases which are of a nature whichlends them to treatment via a retinal prosthetic implant positioned onthe inner retinal surface using the neural contact of the invention. Thefirst disease, macular degeneration, is the leading cause of blindnessin the Western World: age-related macular degeneration affectsapproximately one in ten people over the age of 60 years. Visual lossdue to this disease is progressive, and frequently causes loss in the"legal blindness" range. The pathology of macular degeneration affectsthe rods and cones, as well as a pigmented layer of cells upon which therods and cones are aligned. However, the ganglion cells and theirconnections to the brain remain intact, and being located just below theinner surface of the retina, are opportunely located to be affected byelectrical currents that are applied to the surface of the retina. Aprosthetic retinal implant using the inventive contact may takeadvantage of this arrangement by being placed over and contacting theinner retinal surface for stimulation of those ganglion cells. Ineffect, this scheme bypasses the damaged area of the retina.

The other disease that is treatable with an implant using the inventivecontact is retinitis pigmentosa. The cause of this inherited disease isnot known, but the damage caused by the disease is also at the level ofthe rods and cones. Retinitis pigmentosa results in a progressive lossof vision over decades, leaving many sufferers almost totally blind. Amedical treatment is not available for either retinitis pigmentesa ormacular degeneration.

Also referring to FIGS. 2a and 2b, the neural contact 10 consists of athin cantilever which gently conforms to the curvature of the retinawhile at the same time maintaining low pressure contact with the innersurface of the retina. One portion 20 of the cantilever is physicallyattached to the retina, while the remaining area 26 of the cantilever isheld in contact with the retina by a restoring force which developsalong the cantilever's length-wise axis in response its curvature whenthe cantilever is in place along the inner radius of the retina. Withthis scheme, the area of physical attachment to the retina is distancedfrom the rest of the cantilever, which supports, e.g., stimulatingelectrodes, whereby any damage caused to the neurons by the attachmentwill have no effect on the somewhat remote neurons under the rest of thecantilever--these neurons may be interfaced and, e.g., stimulated. Theattachment portion 20 is thus suited for supporting various circuitry,e.g., data and input power processors, which are not directly involvedin neuron stimulation, while the remaining area 26, being separated fromthis portion, ideally supports the stimulating electrodes forstimulating the retinal ganglion cells.

Depending on the practicality of accessing areas of the inner retinalsurface, the cantilever structure may range from a few millimeters to 5centimeters in length, and preferably between 0.5-2 centimeters inlength. The width of the cantilever may be between 0.5 mm-1 cm, andshould be about 2 mm; this width being determined by the two-dimensionalcurvature of the eye, because the cantilever spring element will onlybend in one direction. The cantilever width does not need to be uniformalong the cantilever length, however. For example, the cantilever mayhave a more narrow central region separating the attachment portion fromthe stimulating region. In addition, appropriate width contours alongthe length of the cantilever could be designed to produce a standardforce magnitude along the length of the cantilever.

The restoring force exerted by the cantilever on the inner retinalsurface is ideally prespecified using an appropriate cantilevergeometry, thickness, and material. Examples of materials suitable forthe cantilever include silicon, silicon nitride, silicon carbide,sapphire, diamond, or other materials which exhibit some flexibility andwhich may be processed to render them biocompatible. In addition, thematerials should be compatible with microfabrication techniques. Thespecific choice of materials will dictate the thickness of thecantilever for providing uniform, low pressure on the retinal surface.If, for example, silicon is used as the cantilever material and siliconeis used to encapsulate the cantilever, the silicon and silicone portionscould both be between 2-40 μm-thick, with the silicon layer beingideally between 5-15 μm-thick and the silicone layer being ideallybetween 5614 25 μm-thick.

Given these geometric guidelines, the cantilever geometry isparticularly specified to provide both an adequate retinal contact and aminimum amount of pressure on the retina. This pressure should ideallybe below 10 mm Hg; the ganglion cells are adversely impacted byprolonged pressures above this level. For example, glaucoma is asignificant cause of blindness which would result from elevation ofintraocular pressure for an extended time.

In particular, the force of the cantilever against the retina should beslightly greater than only the weight of the cantilever assembly. Inaddition, the weight of the assembly is minimized to thereby minimizeboth static and dynamic forces, i.e., accelerations, due to movement ofthe eye. Use of low-density materials such as silicone achieve thisminimized acceleration and also minimize gravity effects on the contactpressure.

The force of the tip of the cantilever opposite the attachment portionmay be determined as follows:

    p=δEbh.sup.3 /4L.sup.2

where

p=force

E=Young's modulus

b=cantilever width

h=cantilever thickness

L=cantilever length

δ=deflection of tip from unbent position

By selection of the cantilever geometry and area, a, the pressure, p/a,may be precisely and predictably specified for a particular material.The shape of the cantilever may be varied to precisely tailor the forcealong the length of the cantilever. This can be done using commonnumerical design modeling and simulation software packages to match thedesign goals of the cantilever with the shape of the eye and thepreferred materials for the cantilever. Because pressure is a functionof applied force per unit area, widening the structure in a particulararea would tend to decrease the pressure in that local area.

As discussed above, the cantilever supports electrodes for stimulatingganglion cells in a location distant from the site of cantileverattachment to the retina. Such electrodes may be of any suitable designwhich would provide electrical current stimuli to the ganglion cellbodies. An array of electrodes may be positioned on one end 26 of thecantilever 10. Each electrode is connected via, for example, conductingtraces, to circuitry 38 located at the attachment end 20 of thecantilever. As discussed above, the circuitry 38 may include pulsegeneration and power circuitry. The circuitry and electrodes may bediscrete electronic pieces which are assembled on the cantilever, orthey may be fabricated as an integrated body with the cantilever. Usinga discrete assembly process, flip chip bonding using one of a variety ofwell-known techniques is preferred because such a method would minimizethe overall mass of the cantilever structure.

Encapsulation of the cantilever, electrodes, and electronics isessential for biocompatibility of the structure with the retinalenvironment. Silicones have been demonstrated as a good choice for anencapsulation material based on their performance during directimmersion in saline environments. Silicones can be mixed to provide awide range of mechanical properties, and can be micromachined in muchthe same manner as conventional electronic materials to a prespecifieddesired geometry. Fluorocarbons and polyesterimides may also be goodencapsulation materials; their use with standard electronic materials,such as silicon dioxide, requires a silane coupling agent that couldcreate stable bonds between these materials and silicon dioxide in anaqueous environment. Other biocompatible materials may also be selectedas an encapsulation material.

As shown in FIGS. 2a, 2b, and 3, the encapsulation layer 24 surroundsthe cantilever structure 40. This layer, preferably of siliconematerial, should be very flexible and soft, and should extend beyond theedges of the cantilever structure 40 by an amount equal to at least itsthickness, and more preferably 4-5 times its thickness. The overhangingarea may be perforated to allow residual vitreous to ooze through andhold it stable once it is in place on the retina. This is particularlyadvantageous as it is crucial to maintain a precise positioning of thecontact structure. The silicone overhang edge also has the advantage ofbeing better matched to the mechanical impedance with the neural tissue,and thereby minimizes the trauma or damage which the structural edgesmay cause to the retinal surface.

Referring specifically to FIG. 2b, the silicone layer is thickest at thepoint of attachment to the retina, and tapers away from the attachmentarea to the electrode area. The encapsulated cantilever structure may beattached to the retinal inner surface via conventional attachmenttechniques, such as using retinal tacks. Other suitable techniques mayalso be employed to physically anchor the attachment portion of thecantilever to the retinal surface.

As shown in FIG. 4, the encapsulation layer includes channels 42 overeach of the cantilever electrodes 28. The encapsulation layer channelsact to recess the electrodes and provide further lateral confinement ofthe electrodes fields, because the encapsulation layer material isinsulating. The encapsulation layer is ideally as thick as the diameterof one electrode, about 25 mm. The recessed design ensures that theequipotential lines corresponding to the electrode electric field areparallel to the surface of the electrode, providing for uniform currentdistribution on the electrode surface. The recess design might alsolimit the accumulation of electrochemical products that may be toxic toneurons and corrosive to the electrodes.

A silicon cantilever structure may be fabricated with the electrodesdescribed above integrated with the cantilever, using standardelectronic fabrication techniques. Beginning with a silicon wafer (notshown), p+ boron is diffused into the wafer in the desired lateral shapefor the cantilever structure 40. The deep p+ layer acts as an etch stopfor bulk micromachining of the cantilever structure once the electrodesare defined on the structure. Such bulk micromachining is preferablyaccomplished using an Ethylene Diamine Pyrocatechol (EDP) selective etchat the end of the fabrication process, as described below.Alternatively, the backside of the silicon wafer may be ground to thinthe wafer prior to the fabrication sequence.

Electrodes and any desired signal processing circuitry may then befabricated on the doped silicon wafer using standard microfabricationtechniques to define the electrode structure, the circuitry components,and electrical interconnections.

Silicone elastomer (for example, Dow Corning MDX-4-4210) is then spunonto the wafer to form a 20-25 μm-thick layer, and is given a minimalcure. Then the silicone is masked and etched to define and expose theelectrode contact opening and device cut out areas. This is achievedusing standard masking and etching processes suitable for silicone. Oncethe patterning is complete, the wafer is then baked thoroughly accordingto the standard cure cycle for the chosen silicone. The entire wafer isthen immersed in standard EDP to etch back the silicon substrate up tothe p+ etch stop. The EDP only slowly degrades the silicone layer. Asindividual cantilever structures break free in the etch bath, they areremoved and cleaned in phosphoric acid to remove any masking layerremaining on the silicone surface.

Many alternate fabrication processes are equally viable, based onparticular choices for the cantilever structural material and theencapsulation material. For example, if silicon nitride, carbide, ordiamond were chosen as the cantilever structural material, that materialwould be deposited on an oxidized silicon wafer via PECVD techniques.Here there is no need for a bulk micromachining etch stop layer. Theelectrode interconnections and electrodes are then fabricated using astandard microfabrication process. Then a 6 micron-thick PECVD siliconnitride layer is deposited on the electrodes. The entire structure ispatterned down to the silicon substrate using standard plasma etchprocesses for the nitride and oxide layers. Silicone is then depositedand patterned as described above. Finally, the entire silicon substrateis etched away in, e.g., EDP, to produce the finished cantileverstructure.

Referring to FIG. 5, the cantilever structure as described above isinherently straight due to the intrinsic mechanical properties of themost suitable materials, like silicon and silicon nitride. However, thecantilever 40 may be prebent into any desired bend or number of bends bycasting a silicone rib into the structure. Alternatively, the structuremay be held in a prebent position using silicone strap structures 56.This scheme is useful to make surgical placement of the structure easierand to avoid "digging" in of the end of the cantilever in the retinaltissue. An important advantage of this feature is that it bends thecantilever so as to contact the retina tissue only at a prespecifiedlocation, e.g., the location of an electrode array, and is out ofcontact over the tissue between the attachment point and the electrodearray. This would be important in applications in which it isundesirable to contact a large area which might compromise regionalblood supply.

In an alternate construction of the cantilever implant, multiplecantilevers may be attached together at one central attachment site,with each cantilever branching from this site. Each cantilever maysupport electronics and electrodes as described above. In anotheralternative scheme, several cantilevers may be individually attached atvarious locations over the inner surface of the retina.

Other embodiments for the low pressure neural contact structure arewithin the scope of the invention. Suitable structure geometries shouldideally be capable of carrying various stimuli or recording electronicsand electrodes. Furthermore, the site of attachment of the structure tothe retina should be distanced from the site of active neural interface.Power and signal processing circuitry, which may mechanically compromisethe neural tissue, should be relegated to the remote attachment site.And optimally, the contact structure should ensure that any stimulatingelectrodes are pressed against the retina with a known degree ofpressure that remains constant despite variations in surgical procedure,despite variations in the strength of the contact attachment to theretina, and despite flexing, deformation, growth and aging of theeyeball.

The low-pressure contact and electrode designs of the invention areuseful for any neural stimulation or sensing application in which neuraltissue surface contact is required. The contact structure provides boththe contacting surface and mechanical support of stimuli and sensoryelectronics and electrodes. Furthermore, the structure provides theability for attachment to neural tissue in a location distant from anactive stimulation location, thereby isolating any physiologicaldegradation due to the attachment. And of equal importance, the contactstructure inherently provides adequate surface pressure for contactingneural tissue, while at the same time minimizing that pressure due toits ability to easily conform to contours in neural tissue.

Thus, the contact structure of the invention is applicable to a widerange of stimulation prosthetics for visual, auditory, and sensorysystems. The contact structure may in fact provide a unitary device forboth stimulation and sensory electronics to stimulate neural tissue andrecord the neural response to that stimulation. The contact may also beused for functional electrical stimulation of the spinal cord neurons,ganglia for bladder activation, dorsal root ganglion neurons, andautonomic nerves such as the vagal nerve, for treating epilepsy, oractivation of paretic limbs, or in treatment for impotence. Furthermore,the contact structure may be used in an implantable system for use inphysiological studies of various sensory systems. The contact may alsobe employed as a sensory system to measure chemical and physicalparameters like oxygen, carbon dioxide, pH, calcium, glucose, ortemperature and pressure, and may provide an implantable mechanism foruse as a chemical dispersement system to disperse chemicals into thenervous system.

We claim:
 1. A low-pressure neural contact structure for contact withneural tissue of the retina within which are ganglion cells to beelectrically stimulated comprising:a first portion for attachment to afirst location on an inner surface of the retina, and a second portioninterconnected with said first portion via an interconnection adapted tocontact a second location on the inner surface of the retina adjacent tosaid ganglion cells to be stimulated, said interconnection easilyconforming to contours in said neural tissue to exhibit a weak restoringforce in response to curvature of the interconnection along an innerradius of the retina to provide a desired pressure of contact of saidsecond portion against said neural tissue of below 10 mm mercury, thedesired pressure sufficient to permit stimulation of the neural tissuewithout damage to said neural tissue.
 2. The neural contact of claim 1wherein said first and second portions together comprise an integralstructure.
 3. The neural contact of claim 2 wherein said integralstructure is of a generally rectangular geometry.
 4. The neural contactof claim 3 wherein said integral structure comprises a cantilever. 5.The neural contact of either of claims 1 or 4 wherein said secondportion provides mechanical support for a stimulating electrodestructure positioned on said second portion for stimulating saidganglion cells.
 6. The neural contact of claim 4 wherein said cantilevercomprises silicon.
 7. The neural contact of claim 6 wherein said secondportion of said cantilever comprises an array of stimulating electrodesfor stimulating said ganglion cells.
 8. The neural contact of claim 7further comprising an encapsulation layer of biocompatible materialwhich encapsulates said cantilever.
 9. The neural contact of claim 8wherein said encapsulation layer comprises silicone.
 10. The neuralcontact of claim 8 wherein said encapsulation layer overhangs edges ofsaid cantilever.
 11. The neural contact of claim 9 wherein saidencapsulation layer material in the location of said first portion isthicker than said encapsulation layer material in the location of saidsecond portion.
 12. The neural contact of claim 10 wherein saidencapsulation layer material in the location of said edges of saidcantilever comprises perforations.
 13. The neural contact of claim 9wherein said encapsulation layer comprises an array of channels locatedat a position over said second portion corresponding to said array ofstimulating electrodes.
 14. The neural contact of claim 9 wherein saidcantilever is between 2-40 microns-thick.
 15. The neural contact ofclaim 14 wherein said cantilever is between 5-15 microns-thick.
 16. Theneural contact of claim 14 wherein said cantilever is between 2 mm-5cm-long.
 17. The neural contact of claim 16 wherein said cantilever isbetween 0.5-2 cm-long.
 18. The neural contact of claim 16 wherein saidcantilever is between 0.5 mm-1 cm-wide.
 19. The neural contact of claim18 wherein said cantilever is between 1-3 mm-wide.
 20. The neuralcontact of claim 14 wherein said encapsulation layer is between 5-25microns-thick.
 21. The neural contact of claim 9 further comprising asilicone rib positioned on said cantilever to bend said cantilever in apredetermined geometric shape.
 22. The neural contact of claim 7 whereinthe width of said cantilever in the location of said first portion andin the location of said second portion is greater than the width of saidcantilever in the location of said interconnection.